Compact gas analyzers, which make possible low manufacturing costs and a robust design because no movable optical components are used, are made available with such optical gas sensors, as they are disclosed, e.g., in U.S. Pat. No. 5,973,326.
The prior-art principle of measurement of the optical gas sensors is based on the concentration-dependent absorption of electromagnetic radiation, specifically in the infrared wavelength range, through the gas to be measured, i.e., the measured gas. The measured gas, e.g., hydrocarbons, CO2 and other trace gases, diffuses, in general, through a dust protection diaphragm or a flame trap in the form of a fabric or a gas-permeable layer of a sintered or ceramic material into the volume of the cell of the measured gas cell of the gas sensor.
The radiation of at least one broad-band radiation source covering, in general, a larger wavelength range passes through the measured gas cell, and an incandescent lamp or an electrically heated glass or ceramic element is usually used as the radiation source. The radiation leaving the at least one electromagnetic radiation source in a divergent form is focused by means of optically reflective surfaces in order to increase the radiation intensity at the site of the measuring detector and possibly of the reference detector. The signal-to-noise ratio of the gas sensor is increased and the quality of measurement is thus improved by the focusing of the radiation. The detectors used are, in general, pyroelectric crystals, semiconductor elements or so-called thermoelectric piles from thermocouples, which convert the measured radiated power into electric signals, which are evaluated for the determination of the gas concentration to be measured in a suitable manner.
If two or more different measured gases are to be measured with one gas sensor, a number of measuring detectors corresponding to the number of different measured gases are used, which are sensitive to the particular measured gas in a wavelength-specific manner. The selection of the wavelength range or wavelength ranges is performed by means of interference filters, arranged upstream of the corresponding detectors, are directly connected to or combined with the corresponding detectors. A wavelength range contains the wavelength of an absorption band of the measured gas and is detected by the corresponding measuring detector, while the wavelength range detected by the optical reference detector is selected to be such that it is not affected by the absorption of the measured gas. The concentration of the measured gas is determined and the effect of aging effects of the radiation source as well as the effect of possible contaminants in the optical beam path are compensated by forming the quotient and a suitable accounting of the measured signals.